It is known to transport and store stacks of goods, for instance bottles or cases of canned foodstuffs, on pallets and to protect and stabilize the palletized stack with a shrink wrap. To this end a lower shrinkable foil is laid atop the pallet, the stack is formed atop this lower foil, an upper foil hood is pulled down over the stack, and the foils are shrunk around the goods.
In a standard prior-art system the hood is gripped at its edge and pulled down over the stack by grippers carried on an annular frame that is vertically displaceable at the wrapping station. Once the hood is in place its edge is released and heater/blowers on the frame are activated as the frame is raised to shrink the wrap and in fact to weld it to the outer edge of the lower foil on which the stack sits, thereby also securing the palletized stack to the pallet. Such an arrangement can trap air in the package and form bubbles as the air is heated. These bubbles do not always recede completely when the package subsequently cools. In addition diagonal folds frequently form at the corners of the package so that the resin of the shrink wrap is insufficiently heated where it is doubled over. What is more the pockets formed by such folds can trap water when the palletized package is stored outside, creating the possibility of leakage and subsequent handling problems. Furthermore as the foil shrinks there is excessive shrinking adjacent the poorly heated doubled areas so that holes can appear at these excessively shrunk regions.
European patent application 205,135 describes a system which avoids some of these problems by holding the bottom edge of the hood out away from the package and by applying the heat from top to bottom while air is aspirated from beneath the package, an effect achieved by releasing the bottom edge of the upper hood foil, raising the frame to its uppermost position, and then activating the heater/blowers as the frame is once again lowered. This system does indeed avoid the formation of bubbles. Nonetheless the upper hood foil frequently shrinks such that it pulls up away from the pallet so that by the time the heater/blowers are in their lowermost position the lower edge of the upper foil is spaced above the lower foil and can no longer be welded to it.